Expanded metal electrical resistance



Oct. 9, 1951 w. F. KNIGHT 2,570,592

y EXPANDED vMETAL ELECTRICAL RESISTANCE Filed Dec. 14, 1948 2 sheets-sheer 2 INVENTOR atented ct. 9, 1951 EXPANDED METAL ELECTRICAL RESISTANCE Walter Frederick Knight, Alfold, England, as-

signor to Exmet Electrical Corporation, New

York, N. Y.

Application December 14, 1948, Serial No. 65,168 In Great Britain July 28, 1948 (El. 20L-69) 6 Claims.

The present invention relates to electrical resistances and to methods of forming the same, More specifically, the invention relates to electrical resistances of the so-called expanded metal type in which the resistance element per se is fabricated from a sheet or strip of flat metal stock by any one of a number of well known processes to produce an article having a medial expanded portion and at least one, but usually two or more selvages of unexpanded solid metal, the latter constituting electrical terminals for the resistance as a whole.

The invention has been illustrated in connection with electrical resistances of this type wherein the selvages, which usually are arranged in opposed relationship at opposite sides of the expanded portion of the resistance, i. e. wherein the resistance is formed from a single sheet of metal stock by a slitting operation whereby only the medial region of the sheet is operated upon by thev slitting knives, while the solid metal selvage is left intact at opposite ends of the sheet or at any desired location or locations. Such illustration of the invention is however` merely exemplary and the same may be employed in connection with al1 manner of resistance devices which employ an area of expanded metal as a path for electrical current, whether the selvages are arranged in opposed relationship at opposite sides of the expanded portion of the device or whether they are otherwise disposed, and also Whether they are integrally formed with the eX- panded metal portion of the resistance or whether they are affixed thereto, as for example, by a welding operation.

Furthermore, while the invention has been illustrated in connection with a resistance element which is three and one-half meshes wide, it will be understood that the resistance may be of any desired mesh width or fractional mesh width without departing from the spirit of the invention.

The term mesh width as employed above may be defined for the purposes of this application as being the width of the resistor ina transverse direction thereaoross as measured by the number of single meshes of the expanded metal that .may be counted along any selected reference line passing centrally through a row of such meshes, this definition being applicable where a mesh is understood to be the generally diamond-shaped area of the expanded metal immediately surrounding any one of the numerous perforations provided in the expanded metal. By the same token, a half-mesh width may be dened as the distance existing from one longitudinal line passing centrally through one longitudinal row of the periorations provided in the expanded material to another line passing centrally through the next adjacent longitudinal row of perforations in which the perforations in this latter row are staggered with respect to the perforations in the former row. By halfmesh length is meant the distance existing from a line passing centrally through a transverse row of the perforations .to another line passing centrally through the next adjacent row of perforations.

Resistances which have been constructed in accordance with the principles of the present invention are suitable for use in connection with motor starting, as for example in connection with the operation of trolley cars, diesel-electric locomotives and the like. The invention is also applicable to the dynamic braking of motors and to the speed regulation of stationary motor installations. Where the resistance is employed as a heater, the same has been found to be suitable for use in connection with both high temperature heaters and low temperature heaters of the domestic type which sometimes are termed convection heaters. The invention is however capable of other uses' and the same may, with or without modification, be employed for a wide variety of uses and, irrespective of the particular use to which the invention may be put, the essential features thereof are at al1 times preserved.

The invention is applicable to all manner of expanded metal regardless of the specific construction of the expanded metal per se. Expanded metal may be manufactured according to a large number of well known processes and it may assume many forms. The invention is thus applicable to the use of what has heretofore and herein is termed intermediate expanded metal as defined in the patent to Hamilton, No. 2,087,- 573 for an Electrical Resistance and Electrical Heater and Method of Producing the Same, wherein there is disclosed a slitted product ob- 3 tained from slitting a sheet of metal in the well known slitting and stretching method of making expanded metal, but with the stretching operation omitted. The preferred forms of the invention illustrated herein are of expanded metal inasmuch as the intermediate expanded type of structure possesses heat dissipating features which are inferior to those of expanded metal.

Finally, the invention is applicable to resist- -ances of the at rectilinear type as illustrated herein as well as to flat resistances having special shapes, and also to those of the split tube or girder type and other special forms of resistances. Irrespective of the particular form which the expanded metal resistance may assume, the principles of the invention remain substantially the same.

It will be appreciated that when an electrical resistance or heater of the type outlined above is in actual use, and particularly where high temperatures are concerned, the temperature rise in the expanded mesh portion of the resistance will naturally be considerably higher than in the unexpanded solid edge or edges, and there is consequently a considerable temperature gradient between each selvage or edge and the expanded metal proper. This temperature gradient is steepest between each selvage and the half-mesh or half-meshes adjoining the same.

Furthermore, the mechanical strength is apt to I be least where the expanded metal joins a selvage and it will be appreciated that temperature expansion tends to cause considerable mechanical stress and consequent mechanical strain at these regions, particularly where the resistor is rigidly supported. Where vibration is an added factor, it is obvious that such stress and consequent strain is materially and proportionately increased.

Owing to these limitations, known resistances of the kind referred to have frequently been unreliable when employed under conditions wherein they are subjected to high peak temperature rises at frequent intervals, as for example when subject to heavy, intermittent and frequent surges of current through the resistance unit. Under these conditions such electrical loading has caused fracturng or rupture of the metal in the regions where the meshes join the selvage edge and furthermore, such rupture of the metal has frequently occurred after only relatively short periods of use.

The present invention is designed to overcome the above noted limitations that are attendant upon the use of expanded metal as resistors or heaters and, toward this end, the invention contemplates, in one form thereof, the provision of an expanded metal resistor wherein the halfmesh length or half-mesh lengths adjacent an inner selvage edge is of greater ribbon width than it is elsewhere in the expanded metal portion of the resistor.

According to another and preferred form of the invention, a gradual diminution of the ribbon widths over several half-mesh lengths removed from the selvage edge is resorted to and, in the preferred embodiment herein illustrated, this diminution takes place throughout two full mesh lengths removed from the selvage inner edge.

By ribbon width is meant the extent of solid metal as a dimension in a direction at right angles to the direction of current flow when the resistor is in use and in any selected half-mesh length or half-mesh width.

fill

In yet another embodiment of the invention, the same limitations that are attendant upon the use of expanded metal as a resistance is overcome by mechanically separating adjacent meshes from each other by a slitting process wherein the metal existing at the juncture between two or more adjacent meshes in the first full mesh row next adjacent an inner selvage edge is cut longitudinally to effectively physically and electrically separate these meshes and thus impart a greater degree of flexibility to the resistor in the vicinity of the selvage inner edge. This form of the invention may be/carried further if desired and still other adjacent meshes further removed from the inner selvage edge may be separated to insure further flexibility, all in a manner that will be described in detail hereinafter.

In the former embodiment of the invention, the limitations induced by electrical loading and consequent stress on the metal in the regions of the juncture between the selvage edge and adjacent half meshes are remedied by affording an increased path for the electrical current flow with a consequent slight decrease in the overall resistance value of the unit as measured in ohms while at the same time increased mechanical strength is attained by the increased margin or area of juncture between the selvage and the mesh portion proper. In the latter' embodiment, the resistance value of the unit remains substantially the same but a greater degree of flexibility tending to resist or counterbalance the resultant strain is resorted to. In both instances the net result which is to materially prolong the life of the resistor under conditions of frequent electrical loading is electively attained.

In still another form of the invention, a combination of the above two methods of minimizing the effect of stress at the lines of juncturel between the selvage inner edge and adjacent half mesh sections or lengths is resorted to. According to this latter form of the invention, not only,

is such longitudinal separation of adjacent meshes resorted to but a gradual diminution in the ribbon widths in a direction extending away from the selvage edge is effected, thus affording both an increased current path as well as greater flexibility.

In connection with this latter form of the invention, the feature of increased mechanical strength at the regions of juncture between the selvage and the expanded metal or mesh portion is preserved but any tendency for stiffness to exist at these regions by virtue of the additional metal thereat is compensated for by the increased exibility offered, this exibility of course being attained by the ability of the adjacent meshes to move or work freely and independently of one another.

The provision of electrical resistances of the various types briefly outlined above being the principal object of the invention, numerous other objects and advantages thereof will become apparent as the nature of the invention is better understood.

In the accompanying two sheets of drawings forming a part of this application, several embodiments of the invention have been illustrated.

In these drawings:

Fig. 1 is a plan view of a preferred form of resistor constructed in accordance with the priny Fig. 3 is a sectional view taken substantially along the line 3-3 of Fig. 1.

Fig. 4 is a fragmentary plan view of another form of resistor constructed in accordance with the principles of the invention.

Fig. 5 is an enlarged sectional view taken substantially along the line 5-5 of Fig. 4.

Fig. 6 is a sectional view similar to Fig. 5 showing a slightly modified form of the invention illustrated in the former figure.

Figs. '7, 8 and 9 are plan views illustrating three additional forms which the invention may assume.

In all of the above described views, like characters of reference are employed to designate like parts throughout. A

Referring now to the drawings in detail and particularly to Figs. 1 to '3 inclusive wherein a preferred embodiment of the invention is shown, a resistance unit is designated in lits entirety at I'U and is preferably in the form of an integral sheet of metal, which may be of a high resistance alloy, having a medial expanded region I2 and opposed end regions or selvages I4 and IB which are of unexpanded solid metal. vage regions I4 and IS may serve as terminals for electrical connection of the resistor in an electrical circuit and also as mountings therefor and, toward this end, these regions may be formed with one or more holes I3 therethrough to facilitate electrical connection and/or mounting of the unit. The solid regions I4 and I6 serve an incidental function in that they lend rigidity or mechanical stiiness to the resistance sheet as a whole.

The expanded portion I2 may be formed according 4to any one of numerous known methods of producing material of this character and these include passing the sheet material between slitting rollers or disks and then stretching it between divergent guides, or they may include the use of serrated reciprocating slitting knives to which the sheet metal is progressively fed by mechanical means in such a manner as to produce by a shearing action a plurality of rows of aligned slits or cuts, the shearing action removing practically no metal in the vicinity of the cuts. The cuts are preferably of uniform length and likewise they are preferably parallel and extend in a direction transversely of the resistor and I parallel to the inner edges of the selvage portions I4 and I6. The cuts in each transverse row are staggered with respect to the cuts in the next adjacent row and the metal in the immediate vicinity and on one side of the various cuts is bent out of the plane of the original sheet in one direction as indicated at 20.

In connection with the extreme marginal rows of cuts, i. e. those cuts which divide the selvage portions I4 and I6 from the expanded metal region I2, the metal on only one side of the cuts is bent out of the plane of the original sheet while the metal on the selvage side of the cuts may remain substantially ilat and in the plane of the -original sheet. The means whereby the selvage portions are maintained substantially ilat in their entirety despite the existence of adjacent offset portions in the expanded part of the resistor forms no part of this invention and no claim is made herein to any novelty associated with the same.

After the metal has been expanded in the medial regions I2, the various cuts above referred to take on a generally diamond shape appearance when viewed in 'plan and aseries of meshes are thus for-med, feach mesh consisting of a `closed parallelogram-like increment of the expanded region :including 'four ribbon-like strips of metal shown at a, b, `c and d in Fig. 1 and in which the parallel ribbon-like strips a. and b are opposed to each other and the ribbon strips c and d are similarly opposed. Each Aribbon strip is common to two adjacent staggered cuts and these ribbon strips when considered collectively afford a series of zig Zag paths from one lselvage region to the other for the ilow of electrical current.

The 'Zig 'zag paths just referred to span various distances although they are all substantially equal in length when considered as a path for the flow 'of electrical current. The shortest of 'these current paths is of half-mesh width. Others may span a full mesh width while still others may span one and one-half mesh widths etc., while 'the widest span extends completely across the 'transverse dimension of the resistance unit. In short, an extremely large number of current paths are available for the current ow and the possible permutations of ribbon strips which cooperate to conduct current from one selvage portion to the other are too numerous to warrant tracing herein.

According to the present invention and at the risk of repetition it is pointed out that in connection with conventional expanded `metal resistors of this type, when such resistors are in actual'use, particularly where high temperatures are involved, a very steep temperature gradient exists in the metal of the resistor at the region where half mesh lengths adjoin the selvage portions. Likewise, the mechanical strength is apt to be least in these regions'.

In order to effectively lessen the steepness of this relatively high temperature gradientras well as to lend additional reinforcement to these regions, it is contemplatedthat in the form of the invention shown in Fig. 1 the ribbon strips designated at e and which are the half mesh ribbon strips directly adjoining the selvage edges be made somewhat greater in width than the width of the more 'centrally located ribbon strips. Such widening of Vthese ribbon strips not only provides an increased path for the flow of current therethrough but also lends Vmechanical strength to the structure at the 'points where it is most needed.

If desired, the ribbon strips f or half mesh sections which adjoin the strips e and which are once removed from the selvage portions I4 and I6 may have a slightly less ribbon width than these former half mesh strips. Similarly, the ribbon strips g which are twice removed from the selvage portions may be of still lesser ribbon width. Likewise, the ribbon strips designated at h which are three times removed from the selvage portions may possess a still further reduced ribbon width and these latter ribbon strips, as well as all of the more centrally located strips may be of equal ribbon width throughout. Such gradual reduction of ribbon width inwardly of the resistance strip has given rise to the coined expression tapered mesh inasmuch as there is a progressive tapering of the ribbon width dimensions proceeding from the selvage portions inwardly of the expanded portion of the sheet.

It will be understood that while this tapering effect has in the illustrated form of the invention of Fig. 1 been carried out throughout two full mesh lengths, obviously this tapering effect may be imparted to any whole or fractional length of the mesh desired.

Another aspect of the invention, apart from considerations of a precautionary nature to prevent overheating at the juncture regions of the selvage and mesh portions of the sheet and to lend additional mechanical strength at these regions, both to prevent rupture of the metal in use as explained above, is the use of the so-called tapered mesh for the deliberate purpose of effecting localized heat distribution. In certain instances it may be desired to construct a resistance unit which is capable of operating in the manner of a heat transfer device to apply a high degree of heat to one region of an object and to apply a lesser degree of heat to another region of the object. The present invention lends itself admirably to such a use and the taper effect outlined above may be applied in an infinite variety O of ways to accomplish this end. For example, in molding press operation it is frequently desired to apply greater heat to the outside portions of the press than to the inside and in such an instance a resistance may be employed wherein the taper effect is extended completely across the mesh portion thereof from one selvage to the other.

It may be stated as a general principle that wherever localized heat generation is required in a particular expanded metal resistance, the decreased cross-sectional area in the ribbon strands in the localized region or regions necessary to increase the ohrnic resistance of the rel sistor at these regions may be brought about by proper application of the tapered mesh principle to prevent burning out of the strands by too abrupt a change in ohmic resistance at or near these regions. Thus the application of the tapered mesh principle, both as regards the provision of an increased path for the flow of current and mechanical reinforcement of the metal at the regions of danger, may take place wholly within the mesh or expanded portion of the resistor as well as in the Vicinity of the selvage inner edge. Furthermore, where any selected area or localized region of the expanded metal mesh is designed by its decreased cross sectional strand area to render a higher thermal output,

the tapered mesh effect utilizing graduated strand widths may be applied on the side of the high temperature region toward which the oncoming electron flow occurs and also on the side from which the electrons leave.

Still referring to Fig. l, it will be noted that the reduction in the relatively high temperature gradient ordinarily existing at the juncture between the selvage edges and the expanded metal portion is attained by affording an increased path for the fiow of electrical current from the selvage portions I4 and I6 into the expanded metal portion. The so-called tapering effect extending over several mesh lengths is restored to in order that there shall not be too abrupt a change in resistance value at any point or region inwardly of the expanded metal portion which would result in a high temperature gradient at this point or region.

Obviously the specific widths of the various ribbon strips e, f, g and lr may vary according to the character of the expanded material, but specific widths that have been found to be successful in connection with several standard types of expanded metal resistors are given in resistance unit the following table wherein the reference characters e, f, g and h refer to those employed in connection with Fig. 1 of the drawings:

. y. Ribbon Width of Graduated R1bb tniry ldth Strands Medial Region ol Resistor (g) (D (C) Inches Inches Inches Inches 64 or 0. 047 0. 059 0. 074 o. 094 Vit or 0. 062 0. O79 0. 099 0. 0125 2t-l 0r O. 078 0. 09S 0. 012% O. 156 z or 0. 094 0.118 0.149 0.188

It has been pointed out heretofore that an almost infinite or at least a very large number of possible permutations may be made to trace the flow of current through the expanded portion of the resistance unit and an inspection of Fig. 1 will reveal that regardless of which of these possible permutations are selected, the tapered effect is not lost and remains substantially the same in each instance. In other words, regardless of the particular path chosen, whether this path be a zig zag one of uniform half mesh width extent, or whether it extend diagonally completely across the full width of the resistance unit, the so called tapered effect as defined above will be apparent and thus a reduction in the temperature gradient at the ordinary point or points of greatest danger will be eliminated.

In Figs. 4 and 5 wherein a modified form of the invention is shown, this form of the invention is predicated upon imparting to the resistance element as a whole a flexibility hitherto unattainable in order that the tendencyr for stress to exist in the regions where the half mesh ribbon strips join the inner edges of the selvage regions when the resistance is in actual use will be minimized, thus prolonging the life of the unit.

In this form of the invention as illustrated in Figs. 4 and 5, similar characters of reference have been applied to the portions or parts of the I0 which correspond to the various portions or parts of the resistance I0 of Figs. l, 2 and 3 to thus avoid unnecessary repetition of description.

Referring now in detail to Figs. 4 and 5, it has been found that this desired flexibility, particularly in the vicinity of the selvage portions I4 and I6', may effectively be attained by separating adjacent full meshes (as defined above) from each other and particularly those meshes which immediately adjoin a selvage inner edge. Thus in the three and one-half mesh wide structure illustrated, the half mesh k is separated from the half mesh l by slitting the metal longitudinally along the juncture line 30. The full mesh m is separated from the full mesh 1L by a similar cut 32, while the full meshes n and o are likewise separated by a cut 34.

The longitudinal cuts 30, 32 and 34 are preferably made by a shearing action which removes practically none of the metal and, in order to effectively separate the adjacent edges of the various cuts, the end edge of one ribbon strip is bent out of the plane of the other as indicated at 31 in Fig. 5 so that subsequent temperature expansion of the metal when the resistor is in use will not cause meeting of the opposed edges.

It is to be noted at this point that after adjacent meshes have been separated by the cutting process outlined above, the separated meshes are free to flex independently of one another so that when longitudinal expansion of the resistor as a whole takes place under the inuence of a temperature rise the thus separated meshes are subject to a crushing or collapsing effect, so to speak, wherein they become elongated in a transverse direction in direct proportion to their tendency to collapse in a longitudinal direction, much in the manner of a lazy tongs mesh.

According to the form of the invention under discussion, only those meshes which adjoin the inner edges of the resistance unit are thus treated to separate the meshes and, in many instances, this will sufce to lend the necessary ilexibility to the structure to prevent rupture of the metal at the danger points or regions previously indicated. However, particularly where the resistor as a whole is comprised of a large number of longitudinal meshes, and thus is subject to relatively large increments of expansion, it may be found desirable to separate the adjacent meshes of such rows as are once removed from an inner selvage edge and, toward this end, additional cuts 36, 38 and 40 may be resorted to if desired.

The net effect of such additional separation of meshes is to distribute the lazy tongs effect into an additional row of meshes to attain greater flexibility as well as to insure proper separation of the adjacent edges of the rst row thereof at all times.

The form of the invention just described and illustrated in Figs. 4 and 5 has been found to be useful in connection `with resistances which are subject to extreme vibrational disturbances in addition to heavy electrical loading and consequent extreme temperature variations.

This type of resistance unit is also useful where the overall length of the resistance is relatively great while the expanded portion thereof is comparatively short for in such instances the relatively large amount of expansion which may be expected cannot readily be assimilated in the mesh portion of the resistance and a proportionately large amount of stress occurs at the regions of juncture between the half mesh ribbon strips and the inner edge of the selvage.

'Ihe distortion of the end 3B (Fig. 5) to cause effective separation between the two adjacent ends of the Various cuts 30 to 40 inclusive may be a natural function of the cutting process since the cutting devices employed, whether they be cutting disks or cutting knives, will ordinarly by their inherent cutting function create such offset cleavage of the metal. If desired however, and as shown in Fig. 6, a small portion of the metal may be removed in the vicinity of each -cut as at 34 and the opposed ends of the adjacent mesh allowed to remain in alignment. Irrespective however of the particular methody employed for effecting separation of the adjacent mesh, the essential features of the invention are at all times preserved.

In Fig. '7, the features of the two forms of the invention illustrated in Figs. 1 and 4 respectively have been in a general way combined. In this form of the invention the resistance unit I consists of a sheet of metal that has been expanded to provide the so-called taper effect of Fig. 1 and it has also been cut or slitted according to the principles of that form of the invention illustrated in Fig. e. This taper effect may exist only in connection with the first transverse row of half mesh ribbon strips e" or it may be carried through rows of ribbon strips which are two, three, four or even more times removed from the selvage portions as exemplified by the portion I 4". Thus in Fig. 7 the ribbon strips designated at e", f, g and h" gradually decrease in ribbon width with all of the medial ribbon strips h .being of the same ribbon width.

Cutting or slitting of the metal to provide separation between adjacent full meshes is likewise an arbitary procedure and will Vary to accommodate different installations. If desired, separation of only the meshes in the row thereof immedialy adjoining the inner selvage edge may be resorted to or separation of the meshes of additional rows further removed from the selvage edge may take place. In fact, the isolated feature of the form of the invention shown in Fig. 1 whereby an increased path for the flow of electrical current is afforded, and the isolated feature of the form of the invention shown in Fig. 4 whereby increased flexibility is provided, may be Varied independently of each other, each with disregard for the other except insofar as such variance mutually contributes to produce an electrical res-istance or heater having predetermined and desired mechanical and electrical characteristics. For example, the so-called taper effect may be carried inwardly of the expanded portion of the resistor throughout four of the half mesh transverse rows but mechanical separation of `the adjacent full mesh portions need not be carried beyond the rst transverse row of meshes adjoining the selvage, or, alternatively it may be carried as far or further than the taper effect is carried. In an extreme case, if desired, the separation of adjacent meshes may be carried completely across the expanded portion of the resistance unit so that the latter is, in effect, divided into a series of one mesh widths, all arranged in physical and electrical parallel.

In connection with the forms of the invention illustrated in both Figs. 6 and 7, the amount of metal removed by the process may vary but in general it is advantageous and economical to remove only that amount of metal which will permit the opposed edges to remain completely separated under conditions of the maximum temperature to be expected when the resistance is in actual use.

In the form of the invention shown in Fig. 8 a similar arrangement to that of Fig. 4 is shown but in the latter form adjacent half mesh portions of the expanded metal are separated from each other by a cutting or slitting procedure. To be more exact, those full meshes 30", 32 and 34" which adjoin the inner edge of the selvage I4 are separated from each other in the manner previously indicated in connection with either Figs. 5 or 6 and, in addition, the meshes which are one and one-half times removed from the selvage edge are also similarly separated. By one and one-half times removed is meant the transverse row of meshes which are staggered with respect to the meshes of the row immediately adjoining the selvage edge and which are shown as being separated by cuts or slits 50". 52" and 54 in Fig. 8.

As is the case with all the previous forms of the invention, the extent to which such cutting and slitting may be carried inwardly from the inner edge of the selvage portions I4 may be varieo.1 to suit engineering exigencies and the mechanical and electrical characteristics desired in the nal product.

In Fig. 9 the invention has been carried a step further and the resistance unit has been modied to produce a so-called taper effect. Thus the ribbon strips e, f", g and hm' gyadually decrease in ribbon width inwardly of the expanded portion of the resistance unit 19, all in a manner previously described in connection with the forms of the invention illustrated in Figs. 1 and 7. Again, this taper effect may be associated with the half mesh ribbon strips immediately adjacent the inner selvage edge or it may be carried further inwardly to any desired degree.

The invention is not to be limited to the exact arrangement of parts shown in the accompanying drawings or described in this specification as various changes in the details of construction may be resorted to without departing from the spirit of the invention. For example, while the invention has been illustrated in several forms thereof in connection with substantially flat resistance elements or units having a medial expanded region and opposed selvage regions, the principles of the invention are applicable to expanded metal resistors of the split tubular or girder type as exemplified by the patent to Melsom, No. 1,991,935, dated February 19, 1935, or, in fact to a large variety of resistors or heaters as additionally exemplified by the patents to Hamilton, No. 2,084,556, dated June 22, 1937, Clay, No. 2,129,046, dated September 6, 1936 and many others.

Only insofar as the invention has particularly been pointed out in the accompanying claims is the same to be limited.

Having thus described the invention, what I claim and desire to secure by Letters Patent is:

1. A metal resistance unit comprising in combination a solid metal selvage region and a contiguous perforate region, the latter being formed with a plurality of rows of perforations extending substantially parallel to the inner edge of said selvage region thereby forming a series of meshes, each of said perforations being surrounded at least in part by solid metal ribbon strips constituting mesh-joining portions, the ribbon width of all such ribbon strips as adjoin said selvage region being greater than the ribbon width of the ribbon strips which are more remote from said selvage portion whereby the mechanical strength of said adjoining ribbon strips and consequently their resistance to distortion due to thermal expansion is increased.

2. A metal resistance unit formed from ila-t metal stock comprising in combination a selvage region having substantial depth and a contiguous expanded region comprised of a plurality of rows i of staggered perforations extending parallel to the inner edge of said selvage region therebyl forming a series of meshes each of said perforations being surrounded at least in part by solid mesh-joining ribbon strips, the ribbon width of such strips as adjoin the selvage region being greater than the ribbon width of such strips as are more remote from said selvage region whereby the mechanical strength of said adjoining strips and consequently their resistance to distortion due to thermal expansion is increased.

3. A metal resistance formed from iiat metal stock and comprising a solid metal terminal portion and an open expanded metal mesh portion having substantial depth and disposed immediately adjoining said terminal portion with the mesh strands of the latter portion merging directly into said terminal portion, such mesh strands as immediately adjoin and merge into said solid terminal portion being of greater ribbon width than the ribbon width of the mesh strands which are once removed from said terminal portion, and the mesh strands which are once removed from said terminal portion being of greater ribbon width than the ribbon width of the mesh strands which are twice removed from said terminal portion.

4. A resistance unit of the expanded metal type comprising a medial expanded portion providing a series of staggered open mesh portions surrounded by ribbon strips constituting meshjoining portions, and a pair of opposed selvage portions constituting electrical terminals for the unit, said selvage portions being integral with said expanded portions and being directly joined thereto by the merging ribbon strips of an adjacent row of half-mesh openings, said latter ribbon strips being wider than the ribbon strips with which they are joined and which are consequently more remote from said selvage edges.

5. A resistance unit of the expanded metal type comprising a selvage portion and an adjacent expanded portion providing a series of staggered open meshes surrounded by ribbon strips constituting mesh-joining portions, the meshes being arranged in rows which extend both longitudinally and transversely of the unit, the ribbon strips immediately adjacent said selvage portion being wider than the ribbon strips which are more remote from said selvage portion, there being a row of half meshes immediately adjacent said selvage portion, adjacent meshes which are one-half mesh removed from seid selvage portion being separated from each other by the provision of longitudinal cuts connecting the half meshes adjoining the selvage portion with the adjacent meshes which are one mesh removed from the selvage portion.

6. An expanded metal resistance comprising medial expanded regions providing open mesh portions surrounded by mesh strands, and a pair of opposed end regions designed as fixed electrical terminals, said end regions being imperforate, the width of such strands as adjoin each end region being greater than the width of such strands as do not adjoin said end regions.

WALTER FREDERICK KNIGHT.

REFERENCES CITED The following references are of record in th iile of this patent:

UNITED STATES PATENTS Number Name Date 918,535 Hadaway, Jr. Apr. 20, 1909 1,267,978 Collinson May 28, 1918 1,313,028 Thomson Oct. 7, 1919 1,332,030 Collinson Feb. 24, 1920 1,396,871 Nagel Nov. 15, 1921 FOREIGN PATENTS Number Country Date 317,517 Great Britain Aug. 16, 1929 345,138 Germany Dec. 5, 1921 369,231 Great Britain Mar. 16, 1932 

